1. Technical Field
The present invention relates to power-save circuits, and more particularly to a power-save circuit capable of saving energy when a computer is it off state.
2. General Background
In a personal computer (PC) system, power management is adopted to conserve energy while the PC is in use and put the PC to sleep to save energy when the PC is not in use. System power states derive from the Advanced Configuration and Power Interface (ACPI) specification. They are defined as follows:
S0/Working—The CPU is fully up and running; devices are powering up and down as needed.
                S1—The CPU is stopped; RAM is refreshed; the system is running in a low power mode.S2—The CPU has no power; RAM is refreshed; the system is in a lower power mode than S1.        S3—The CPU has no power; RAM is in slow refresh; the power supply is generally in a reduced power mode (for example, it can't supply much power and is running in a lower power efficiency mode).        S4—The hardware is completely off; system memory has been saved to disk.        S5/Off—the hardware is completely off; the operating system has shut down; nothing has been saved.        
When the computer is powered off and at S5/off state, there is still a +5V standby voltage (+5VSB) supply applied to a motherboard for driving a basic power source control circuit of the computer system during the off state. The control circuit provides advantages. For example, the system can be preset to be turned on automatically at night. Therefore, an international facsimile can be sent during a period of time that has a cheaper calling rate without the presence of an operator. Moreover, while no operator is available or present, the system can be turned on by an incoming event such as a signal received by a modem. The system can thus receive the incoming message or document automatically. However, this means the computer still consumes energy even in the off state.
What is needed, therefore, is power-save circuit capable of decreasing unnecessary power consumption when a computer is in an off state.